76 MICROSCOPIC ILLUSTRATIONS. 



natural size and proportions, taken at the same period 

 of its growth as that at which the drawing was made. As 

 it approaches maturity, it casts its skin several times, 

 from each of which it escapes by a rent formed down 

 the back. 



The large tracheae, or air- vessels, which run along each 

 side of the body, with the numerous branches emanating 

 from them at various intervals, in their course, are com- 

 posed of delicate transparent membranes, distended by 

 means of fibres attached to, and wound about them in a 

 spiral form, like the thread of a screw or the spring 

 of a bell. The diameter of these vessels, one of which I 

 dissected from a larva nearly an inch and a half in length, 

 is one-sixteenth of an inch. When mounted between two 

 pieces of glass, and submitted to moderate amplification, 

 they exhibit the most beautiful and varied specimen of 

 line-work that it is possible to imagine. The fibres of 

 the upper and under sides intersecting each other at dif- 

 ferent angles, produce an effect which no engine-turner 

 could surpass; it would, therefore, be useless attempting 

 to illustrate it by a. wood engraving. A branch of this 

 vessel may also be observed running along each of the 

 legs. 



Respiration by this creature is not performed as with 

 the larva of the ephemera, although, like it, it is an inha- 

 bitant of the water. The tracheae in the latter is supplied 

 with air from the membranous paddles on each side of 

 the body, which imbibe it from the circumambient fluid. 

 In the larva under consideration, the air is supplied by 



